The large threshold drop in early dark adaptation appears to reflect the decay of the receptor potential to the adapting light. There appear to be considerable differences between the temporal characteristics of the threshold-drops of individual subjects; however, this is a result that is hard to reconcile with the theory, which is stated in terms of cellular mechanisms. During the coming year the extent of variation between subjects will be specified quantitatively to determine the seriousness of the difficulty for the theory. A retinal densitometer of the Rushton-Hood "Florida State" pattern is being modified to permit measurements of cone pigment changes during light adaptation. The device permits measurements of thresholds, as well as pigment concentrations, using the same conditions of measurement. It is intended to provide a direct assessment of the contribution of non-photochemical factors to visual sensitivity. Efforts have been made in the past to make such comparisons for dark adaptation, but not with identical parameters of measurement, and not during light adaptation.